The field of the invention relates to constant tension devices, particularly as used in conjunction with filter bags.
Electric utilities and industrial companies have been largely responsible for the increasing use of fiberglass filter bags for removing particulates from boiler flue gas. As discussed in an article entitled "Make fiberglass filter bags last longer by maintaining proper tension" in the March, 1980 issue of Power, it has been found that premature bag failure can be avoided with proper and uniform tensioning methods. Insufficient tension contributes to abrasion, flex failures, and plugging of the fabric. Loose bags tend to rub against each other or against baghouse walls and results in surface abrasion. Flex failures occur due to the bending or flexing of the bottom portion of the bag. If the insufficient tension inhibits reverse air flow, the bag may not be cleaned properly and will remain plugged.
Excessive tension causes failures in fixed or anchored portions of the bag. Openings may develop in the fabric causing the high-velocity flue gas to sandblast adjacent bags.
It has been found that although a bag may be properly tensioned initially, the tension will decrease as the bag tends to elongate. An 11.5-inch-diameter bag will be subjected to a tension of 70 to 75 pounds. Due to this tension, the bag will increase in length due to some elongation of the fibers themselves, thermal expansion or fabric take-up. Since the bags may commonly be 20-30 feet in length, elongation of several inches may be expected.
Conventional filter bag tensioning devices utilize springs which, because of their non-linear force diagrams, make it impossible to maintain a constant tension on the bag. Instead, as the bag material creeps or stretches, tension is reduced. On retensioning, due to the high spring rates employed, the tension is sometimes increased to a point which causes excessive creep or stretch. A subsequent reduction in bag life and performance accordingly results.
The conventional tensioning spring is typically positioned between a support within the bag house and a bag cap secured to the top of the filter bag. A cable or chain may be employed for securing the spring to the cap and support. The opposite end of the bag is secured to a lower support of the baghouse.